To create a simple script in Photoshop follow these steps:

1. Create a file [name].jsx and put it to this folder

C:\Program Files\Adobe\Adboe Photoshop CS5\Presets\Scripts

2. Open Photoshop and go to File -> Scripts. There will be a new script with the [name].

3. If you need assign it a shortcut. Go to Edit -> Keyboard Shortcuts (Ctrl+Shift+Alt+K) and navigate to File -> Script -> [name] in the dialog.

4. Open the script file and put the JavaScript code to it

var doc = app.activeDocument,
w=850,
h=doc.height

doc.crop(Array(0,0,w,h))

This script crops current image to a fixed width and saves its original height.

5. Save the file and run script from File -> Scripts -> [name] or with a shortcut.

Additional Documentation